1. Field of the Invention
This invention is directed to adaptive filters and adaptive arrays, in general, and to an adaptive filter or adaptive array having significantly improved operating characteristics, in particular.
2. Prior Art
There are many types of adaptive filters and adaptive arrays known in the art. These adaptive filters and adaptive arrays can be used in a number of applications such as in noise cancellation, line enhancement and sidelobe cancellation or the like. Typically, the noise canceller application of an adaptive filter is used in order to separate a primary signal from a noise signal. The input signal which comprises both the desired signal and the noise signal is applied to a circuit and a reference input signal is supplied thereto as well. The filter is operative to effectively subtract, or remove, the reference (or noise) signal from the primary input signal, thereby leaving only the desired signal as an output. The output signal is returned to the adaptive filter portion to alter the operation thereof so that an improved output signal is provided. The most important prior art applications use the so-called LMS (east Mean Square) algorithm to control the operation of the adaptive filter. Other algorithms have been proposed but are, generally, very expensive in hardware or have poorer performance in terms of convergence rate or residual error. Moreover, some of the proposed algorithms or techniques are of marginal utility. Most of the known devices exhibit poor performance in terms of the convergence rate or the residual error.
In the case of the line enhancer, a single input signal including desirable and undesirable components is provided. The reference input signal for the adaptive filter input is derived by delaying the input signal thereby decorrelating narrow band and wide band signals. The output signal from of the adaptive filter is then combined with the input signal to produce an error signal which is returned to the adaptive filter for alteration of the operating characteristics thereof to enhance the narrow band signal at the adaptive filter output which is then subtracted from the input signal to produce an enhanced broadband output signal.
In the case of the adaptive antenna, two major input signals are provided. One input is the output of the main antenna array which contains the signal and an unwanted noise or jamming signal. The other input is an independent or reduced directivity input in which the unwanted-noise-to-desired-signal ratio is much greater than the jamming-to-signal ratio in the main beam. The error signal is formed by subtracting the unwanted noise from the main signal plus noise. This error signal is used to adjust the weights W.sub.1, . . . , W.sub.L such that the error signal power is minimized. This minimization insures that the unwanted noise (or jamming signal) has been removed from the main signal channel to the maximum possible extent.
In the prior art devices, the algorithm which operates on the error signal and intermediate filtered signals to permit convergence on the minimum signal is undesirably slow. Contrariwise, if a relatively rapid convergence is desired and selected, the residual error is frequently relatively large and beyond the desired limits. Thus, relatively slow operation permits a rather accurate convergence and a relatively low residual error, while rapid convergence produces a large residual error. The tradeoffs in terms of time and accuracy are significant.
It is also known to use finite impulse response filters (FIR) or infinite impulse response adaptive filters (IRR). However, it is typically found that the convergence on the IIR filter is extremely slow when there is a high nose factor (i.e., low signal-to-noise ratio). That is, these filters are limited by the fact that low residual error is desirable which, typically, requires a low convergence rate.